The fluoran compounds represented by the above described general formula are synthetic compounds obtained by the present inventors, and although they themselves are usually faintly colored crystals they are characterized by that when their solutions dissolved in an organic solvent are brought in contact with an electron receptive color-developing agent (hereinafter abbreviated color-developing agent) such as active clay or phenolic-resin, the lactone ring in said compounds opens and immediately produces green, black, or greenish-black color. They are therefore very useful as electron donative color-forming agents (hereinafter abbreviated color-forming agents) for the manufacture of recording materials.
As is well known, recording materials utilizing color-forming agents include pressure-sensitive recording paper, heat-sensitive recording paper, electrothermal recording paper, hectograph transfer paper, and the like.
A number of compounds have been proposed as color-forming agents including various lactones, including crystal violet lactone and malachite green lactone. In addition, thiazine compounds such as benzoyl leucomethylene blue have been proposed, as well as leucoauramine and spiropyran compounds. Various problems have been associated with prior art color formers and, particularly in view of the increased importance of pressure sensitive copying papers in the business world, the search for improved color-forming agents has been widespread. Although the colors provided by the prior art color-forming agents have been generally satisfactory, a particular problem has arisen in that the copy papers often are unsuitable for permanent copies. This is particularly true for color-forming agents which produce greenish-black colors. A cause of this problem is the fact that prior art color formers do not provide colors which have a sufficient degree of fastness to the effects of light. In addition, color-forming agents have been found to be poorly soluble in certain important solvents.
As examples of prior art compound which have been the object of research reported in recent United States patents may be mentioned the work reported by Sullivan, U.S. Pat. Nos. 3,244,548; Kimura et al, 3,501,331; and Katayama et al, 3,514,311 Fluoran dyestuffs of a different series, being totally devoid of substituents other than hydrogen at the 5- and 6-positions, are reported as the object of research in German Offenlegungsschrift No. 2,024,859, opened Nov. 26, 1970.
Among the color-forming agents of the prior art, the lactones have generally been utilized, as they develop into a wide range of colors, from yellow and red to blue and green, at a high developing speed. Particularly with respect to color-forming agents which develop into a greenish-black, consideration is directed to the lactones, and more particularly to the fluoran compounds for several practical reasons. Greenish-black color formers have great significance with respect to the purpose for which pressure sensitive copying papers are employed. Most of the pressure sensitive copying papers now in use employ either crystal violet lactone or benzoyl leucomethylene blue as the principal color formers, both of which develop into blue; the resultant blue-copied image is not suitable for recopying by xerography or a diazo system. This problem can be solved by obtaining a black-copied image through (1) the employment of a black color former, or (2) the combined employment of two or more color formers to synthesize a black copied image. Fluoran compounds which develop into a greenish-black therefore are well-suited for such recopying purposes. A color-forming agents which develops into a black shade can achieve the purpose by itself and one which develops into green can produce a stabilized and satisfactory black copied image when mixed with another color former which develops into red.
Conventional greenish-black fluoran color-forming agents have not proven to be satisfactory in that they are poorly soluble in aromatic-aliphatic mixed solvents. Specifically, aromatic solvents such as diphenyl chloride have been commonly used for dissolving the color-forming agents for pressure sensitive copying paper. Recently, however, for economic and other reasons, aliphatic solvents such as high boiling petroleum fractions are admixed into the solvent, thus requiring that the color-forming agents have a solubility in such aromatic-aliphatic mixed solvents.
The fluoran compounds of the invention also can be easily synthesized by the process which comprises contacting an acylaminophenol of the formula ##STR2## with a halophenyl compound of the formula ##STR3## and deacylating the resultant intermediate to produce a diphenylamin of the formula ##STR4## thereafter condensing said diphenylamine with o-[4-di(lower alkyl) amino-2-hydroxybenzoyl] benzoic acid wherein R and R.sub.6 are lower alkyl, hal is halogen, alk is alkyl, n is an integer of 1 to 5 and R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are as defined above. In a preferred embodiment, hal is iodo or bromo.